Radiation and its properties

Radiation and its properties

1. CONTENTS :- DEFINITION OF RADIATION SPECTRUM OF ELECTROMAGNETIC REDIATIONS. SURFACE EMISSION PROPERTIES. ABSORPTIVITY, REFLECTIVITY AND TRANSMISSIVITY BLACK BODY OPAQUE BODY WHITE BODY GRAY BODY

2. RADIATION:- The transfer of energy across a system boundary by means of an electromagnetic mechanism wich is caused solely by a temperature difference. Radiation heat transfer does not require a medium. The radiant heat exchange between two bodies depends on the difference between their temperature to the Fourth power. E=σT 4 W/m 2 E=σAT 4 watt

3. The Electromagnetic Spectrum The electromagnetic (EM) spectrum is the range of all types of EM radiation. Radiation is energy that travels and spreads out as it goes – the visible light that comes from a lamp in your house and the radio waves that come from a radio station are two types of electromagnetic radiation. The other types of EM radiation that make up the electromagnetic spectrum are microwaves, infrared light, ultraviolet light, X-rays and gamma-rays. Radio: Your radio captures radio waves emitted by radio stations, bringing your favorite tunes. Radio waves are also emitted by stars and gases in space. Microwave : Microwave radiation will cook your popcorn in just a few minutes, but is also used by astronomers to learn about the structure of nearby galaxies. Infrared: Night vision goggles pick up the infrared light emitted by our skin and objects with heat. In space, infrared light helps us map the dust between stars. Visible: Our eyes detect visible light. Fireflies, light bulbs, and stars all emit visible light. Ultraviolet: Ultraviolet radiation is emitted by the Sun and are the reason skin tans and burns. "Hot" objects in space emit UV radiation as well. X-ray: A dentist uses X-rays to image your teeth, and airport security uses them to see through your bag. Hot gases in the Universe also emit X-rays. Gamma ray: Doctors use gamma-ray imaging to see inside your body. The biggest gamma-ray generator of all is the Universe.

4. SURFACE EMISSION PROPERTIES. The rate of emission of radiation by a body depends upon the following factors:- Temperature of surface. The nature of surface, and the wavelength or frequency of radiation. The parameters which deals with the surface emission propertiesare given below:- Total emissive power:- Total amount of radiation emitted by a body per unit area and time. E b =σT 4 W/m 2 E b =σAT 4 watt σ=Stefan-Boltzmann Constant=5.67x10 -8 w/m 2 K 4 2. Monochromatic emissive power:- The rate of energy radiated per unit area of the surface per unit wavelength. 3. Emission from real surface emissivity:- It can be calculated by. E b = ἐ σAT 4 ἐ =Emissivity of the material=Ability of the surface of a body to radiate heat.

5. ABSORPTIVITY, REFLECTIVITY AND TRANSMISSIVITY Let, G=Incident radiation G r =Reflected radiation G t =Transmitted radiation G a =Absorbed radiation By the conservation of energy principle G a +G t + G r = G Dividing both sides by G, We get INCIDENT RADIATION REFLECTED RADIATIONS TRANSMITTED RADIATION ABSORBED RADIATIONS

6. BLACK BODY B lackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. For a black body, τ = 0, α = 1, and ρ = 0. Planck offers a theoretical model for perfectly black bodies, which he noted do not exist in nature: besides their opaque interior, T hey have interfaces that are perfectly transmitting and non-reflective.

7. OPAQUE BODY When no incident radiarions is transmitted through the body, It is called an “ Opaque Body ” That is, τ = 0 and α + ρ = 1. INCIDENT RADIATION REFLECTED RADIATIONS ABSORBED RADIATIONS

8. WHITE BODY If all the incident radiation falling on the body are reflected , It is called “ White Body ”. That is, τ = 1 and α = ρ = 0. Type of reflection Regular reflection diffuse reflection INCIDENT RADIATION REFLECTED RADIATIONS

9. GRAY BODY A grey body is defined as a body with constant emissivity over all wavelengths and temperatures. or A grey body is one where α, ρ and τ are constant for all wavelengths.